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Chemical Periodicity History of the Periodic Table * Dobereiner - Law of Triads Early in the 19th century, scientists began to seek ways to classify the elements. One attempt was by Johann Dobereiner, a German chemist, in 1817. Dobereiner found that properties of calcium, barium, and strontium were very similar. He also noted that the atomic mass of strontium was about midway between those of calcium and barium. He grouped these three elements into what he termed a triad . Later, Dobereiner found several other groups of three elements with similar properties. In each case the middle element has an atomic mass about halfway between those of the first and third elements in the triad. Dobereiner's ideas were not taken seriously by others - they looked like coincidences, but the idea of trends had begun. Triad 1 Triad 2 name atomic mass name atomic mass first element calcium 40.1 lithium 6.9 third element...

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